I have been repairing the worn-down silver trim on a number of the Classic models in my collection, here is what I learned.
A) Where to find the paint for the repair
My first visit was a model shop, thinking they would have the best paint selection. They have a good selection with expert advice, but I found them to be no match with craft shops like Hobby Lobby. I heard art supply stores are also a good source.
B) Paint applicator
I tried the finest brush, Q-tip squeezed into cone-shape with a sharp point (a model trick), pointed paint pen, and chiseled paint pen. The chiseled paint pen is by far the best method, giving consistent smooth lines.
C) Testors paint
I first tried Testors silver enammel paint. I found the silver to be a little dull, and it rubs off even after a day of drying. You need to clear coat it to prevent silver fingers.
D) The best solution I found (so far)
I came across a paint pen called "Silver Leafing Pen" by Krylon. It has a chisel point, which makes application extremely easy and even. The paint has a shinier apperance, a better match to the chrome than Testors. Best of all, after a day of drying, it stays on much better. No silver finger, making the clear coat optional. For $5, it is by far the best solution I found so far.
E) Other options I have not tested
I also saw chrome paint in a spray bottle, it is supposed to give an even closer match to the original chrome. But you would have to spray it into a small cup, then apply it with an applicator of your choice. Besides, I do not know if it will react with the hard plastic, so I never explored that option.
Hope you find the information useful in restoring your Classic HPs.